T-tail configuration aircraft have a vertical tail surface called a vertical stabilizer extending upward from the rear end of the aircraft fuselage and have a pivotally movable horizontal stabilizer extending laterally from a point near the top of the vertical stabilizer. The horizontal stabilizer in this configuration is referred to as a flying wing in some publications.
A horizontal stabilizer is used to vertically trim aircraft in flight and to control the pitch, that is, the nose up or nose down, physical orientation of the aircraft on takeoff and landing. The T-tail configuration is used on twin engine aircraft when the engines are located at the rear of the fuselage to avoid the jet discharge.
Recent aircraft engine design improvements have required the positioning of the engines at the rear, higher on the fuselage than previous configurations. This has had an effect on the design considerations for a flying wing. When the engines are located high on the fuselage, the thrust of the engines create a dive tendency to pitch the aircraft downward because the engines are located above the vertical center of gravity of the aircraft. This effect is more pronounced at takeoff and landing when the engines are at maximum or near maximum thrust and air speed is low. At low air speeds, less air moves across the horizontal stabilizer making it have less effect than at higher air speeds.
To overcome these problems the designer must either increase the size of the horizontal stabilizer or increase the amount of rotation of the pivotally movable horizontal stabilizer so that the horizontal stabilizer can trim the aircraft as necessary. In either case the amount of downward rotation required for the flying wing will be dependent upon the amount of movement needed to counteract the aforementioned dive tendency that results from the required positioning of newer aircraft engine designs.
In a T-tail configuration the support structure for the movable horizontal stabilizer protrudes through the vertical stabilizer. This support structure which passes through the vertical stabilizer is mounted within the vertical stabilizer by a hinge means which permits pivotal movement upward and downward of the horizontal stabilizer substantially transverse to the vertical stabilizer. For the purpose of clarification in this Specification, an upward or downward movement of the horizontal stabilizer is in reference to the leading edge of the horizontal stabilizer. A vertical stabilizer structural opening is necessary to permit the angular movement of the support structure as it pivots about its hinge. Air flow around these structural openings can be detrimentally disturbed when these structural openings are exposed, reducing the aerodynamic effectiveness of the horizontal stabilizer and the vertical stabilizer.
There are two main methods which have been used to cover structural openings which are needed to mount a pivotally movable airfoil. One method is to use a deformable seal to cover the opening, and the other uses externally mounted fairings in various arrangements to cover the opening. A principle consideration of both methods is to provide an effective aerodynamic seal for the structural opening with as small an impact as possible on the aerodynamic performance on the aircraft, that is, to keep additional weight and aerodynamic drag to a minimum when adding the aerodynamic seal to the structural opening.
Fairings have long been used to cover structural openings provided on an aircraft in order to mount a horizontal stabilizer. Fairings, called cover plates, are attached to and movable with the horizontal stabilizer. These cover plates are located above and below the horizontal stabilizer and are substantially larger than the structural opening which they cover. The sizes and shapes of the cover plates are determined by the size of the openings, the total pivotal movement of the horizontal stabilizer and the shape of the horizontal stabilizer support structure. The edges of the cover plates encompass the structural opening and make contact with the opposite adjacent surface on the vertical stabilizer or fuselage as they wipe across that surface through movement of the horizontal stabilizer. The contact between the cover plates and the opposite adjacent surface forms the aerodynamic seal.
The requirement that cover plates have an opposite substantially flat surface area to wipe against is the limiting factor in their use to cover structural openings. The two locations on an aircraft which would be subject to these limitations are on a curved surface, such as on a fuselage, or near the edge of structural surface, such as the top of a vertical stabilizer.
The limitation of a curved surface can be overcome by the addition of fixed fairings to the curved surface to form a substantially flat surface area for the cover plate to wipe against. However, this adds to the weight of the aircraft and increases the aerodynamic drag.
The limitation of placing a structural hole near the edge of a structural surface can be overcome by the addition of a fixed fairing to the surface which would extend the surface area, for example by increasing the height of a vertical stabilizer so that the top cover plate does not extend beyond the top of the vertical stabilizer into the airstream. This method also adds weight and increases aerodynamic drag.
One solution to the problem of a structural hole near the edge of a surface can be found in Ridley, Jr., et al, U.S. Pat. No. 4,034,939. This assembly does not use cover plates, but uses a deformable seal which allows the seal to compress and contract as the horizontal stabilizer is actuated between its maximum up and down positions. However, for this arrangement to be effective, the horizontal support structure hinge must be located such that the structural opening needed for up and down pivotal movement is symmetrical. To achieve this symmetry, the hinge for the horizontal stabilizer support structure must be mounted at the center of the structural opening instead of allowing a remote axis of rotation as sometimes preferred by aircraft designers because this configuration allows a wider distribution of load forces. In addition, the increased downward movement of the horizontal stabilizer, as required for some newer engine designs, require larger structural openings which may be too large for this type of deformable seal to accommodate.
Another solution to the problem is taught by Backlund, et al, U.S. Pat. No. 3,756,529, assigned to the assignee of the present invention. This arrangement teaches that the opposite surface area needed for the contact area of the cover plates can be reduced by the use of pivotally mounted, spring loaded doors hinged to the edges of the structural opening to form a continuation of the substantially flat surface. This allows the structural opening to vary in size depending upon the orientation of the horizontal stabilizer. By this method the top and bottom cover plates can be made smaller. Cam members are fixed to the horizontal stabilizer for pivoting a door inward when the horizontal stabilizer needs to rotate into the area of the structural hole normally occupied by that door.
As previously mentioned newer designs require a greater downward movement of the horizontal stabilizer, therefore, fixed fairing extensions to keep the top cover plate out of the airstream when the horizontal stabilizer is pivoted to the full up position must be made taller. The height of vertical stabilizer fairing extension can be reduced by the arrangement as taught by the U.S. Pat. No. 3,757,529 assembly.
However, this prior art arrangement does not present the best solution to the problem. The pivotal door does occupy space, even in a folded position, and therefore the structural opening must be cut larger than just the area necessary for the maximum up and down pivotal movement of the horizontal stabilizer. In addition, the pivotal door adds to the height of the vertical stabilizer because the door must be mounted high enough to provide a clearance between the folded door and the horizontal stabilizer when it is pivoted through its maximum upward movement. This arrangement also contributes to the aerodynamic drag on the aircraft because it uses the traditional approach of an externally mounted cover plate to make contact with the external surface of the aircraft, and therefore the external cover plate is in the airstream. Additionally, this arrangement is an aerodynamic seal which is composed of two distinct elements, a pivotal mounted door and a cover plate. Each must be manufactured, installed and maintained to work in combination with the other, increasing the overall cost of providing an aerodynamic seal for a T-tail configuration.